1. Field of the Invention
The present invention relates to a thin film transistor (TFT) using an oxide semiconductor layer, and a display device and an electronic device, each of which uses such a thin film transistor.
2. Description of Related Art
In a flat panel display such as a liquid crystal display or an organic EL (Electro Luminescence) display, a silicon (Si) material is typically used for a channel layer (active layer) of a driving thin-film transistor. Specifically, for example, amorphous Si or polycrystalline Si is used.
However, in the case of using amorphous Si for the channel layer, while a display is easily increased in size, since carrier (for example, electron) mobility is low, about 1 cm2/V*s or less, display performance has been hardly improved. On the other hand, in the case of using polycrystalline Si for the channel layer, while carrier mobility is high, about 30 to 300 cm2/V*s, a display has been hardly increased in size due to large in-plane variation of transistor characteristics.
From these, to achieve large size and high performance of a flat panel display, a channel material, having high in-plane uniformity similar to that in the case of using amorphous Si and high carrier mobility similar to that in the case of using polycrystalline Si, has been demanded to be developed.
Thus, as a channel material that may meet such a demand, oxide semiconductor such as zinc oxide or indium-gallium-zinc oxide (InGaZnO) has been proposed, and particularly amorphous InGaZnO4 is being widely investigated (for example, see Japanese Unexamined Patent Application Publication No. 2007-281409).